Electrosurgery involves applying relatively high voltage, radio frequency (RF) electrical power to tissue of a patient undergoing surgery, for the purpose of cutting the tissue, coagulating or stopping blood or fluid flow from the tissue, or cutting and coagulating the tissue simultaneously. The high voltage, RF electrical power is created by an electrosurgical generator, and the electrical power from the generator is applied to the tissue from an active electrode manipulated by a surgeon during the surgical procedure.
The amount and characteristics of the electrosurgical power delivered to the patient is determined by the type of surgical procedure performed and the amount of electrosurgical output power required, as well as the tissue characteristics of the patient. Selecting the cutting mode of operation causes the electrosurgical generator to continuously deliver relatively high RF power of moderate voltage. Selecting the coagulation mode of operation causes the electrosurgical generator to repetitively deliver relatively short bursts of high voltage, resulting in a relatively low average output power delivery. Selecting the “blend” mode of operation causes the electrosurgical generator to deliver output power having characteristics which are related to both cutting and coagulation. The blend mode of operation involves repetitively delivering relatively longer bursts of somewhat lower voltage RF output power, resulting in a relatively moderate average output power delivery. In the cut mode, for example, the continuous power output may be as high as 300 watts with an open circuit output voltage in the neighborhood of 2,000 volts peak to peak. In the coagulation mode, the bursts may reoccur at a frequency of approximately 30 kHz, have a time duration of approximately 3 microseconds, and have a peak to peak voltage of approximately 10,000 volts. A typical blend mode will involve bursts at the same frequency of approximately 30 kHz, but with time duration of approximately 5-7 microseconds and at a peak to peak voltage of approximately 4,000 volts. The higher voltage required for coagulation and blend is necessary to cause longer arcs of electrical power to jump from the active electrode to the tissue. Lower output voltage is used for cutting because long electrical arcing is not as important or necessary for cutting.
The electrosurgical generator should also have the capability to deliver these types of RF electrosurgical power under a wide variety of different and rapidly changing output conditions. The impedance of the tissue into which the RF output power is delivered may change substantially from point-to-point as the active electrode is moved during the surgical procedure. For example, a highly fluid-perfused tissue such as the liver may exhibit a resistance or impedance in the neighborhood of 40 ohms. Other tissues, such as skin which has very little moisture content, or the marrow of bone because of its physiology, have an impedance in the neighborhood of 1000-2000 ohms. Average tissue impedances range in the neighborhood of approximately 500 ohms, although the fat or adipose content of the tissue increases its impedance.
The power transfer or delivery capabilities of an electrosurgical generator, like any other power amplifier, depends on the output load characteristics into which the power is transferred. The most efficient power transfer occurs when the internal impedance characteristic of the power amplifier is matched to the external impedance into which it delivers power. Since the internal impedance characteristic of the usual electrosurgical generator cannot be matched to the widely varying tissue impedance into which the electrosurgical power must be transferred, the electrosurgical generator should have the capability to deliver relatively higher amounts of power to compensate for the usual mismatch between the internal generator impedance and the widely varying values of the external tissue impedance, and to do so on an almost instantaneously changing basis as the surgeon moves through and works with the different types of tissues at the surgical site.
Further still, an electrosurgical generator must deliver the RF electrosurgical power under tightly regulated and precisely controlled conditions. Any attempt to meet the rapidly changing power requirements cannot be accompanied by output RF electrosurgical power which causes damage to the tissue or injury to the patient or surgical personnel. Rapid and reliable control over the delivered power is essential to safe and dependable performance of the surgical procedures. Very few, if any, electrosurgical generators have the capability to meet all of these requirements, regardless of how well these requirements are understood. Indeed, almost no other electrical amplifier or power supply is subject to such widely varying requirements.
Most electrosurgical generators have relatively fixed operational features with only traditional and somewhat limited functional capabilities available for selection and use by the surgeon. For example, almost all electrosurgical generators permit the surgeon to choose output delivery characteristics which will accomplish cutting, coagulating or blended operation, and almost all electrosurgical generators permit selection and adjustment of the amount of power to be delivered in each of the selected modes of operation. However, beyond these traditional options, most electrosurgical generators do not have the capability to perform newly-developed specialized procedures which may require delivery of an electrosurgical output waveform having somewhat different characteristics than those available from a standard electrosurgical generator. Instead, limited use electrosurgical generators having such specialized output power characteristics are usually developed specifically for such procedures. On the other hand, specialized generators are generally not capable of more generalized surgical performance required from a more conventional electrosurgical generator, so the specialized generators cannot be used as substitutes of the general purpose generators.
Because the cost of an electrosurgical generator is significant, most hospitals and surgical operating facilities have only general-purpose electrosurgical generators on hand for surgical procedures. Once acquired, an electrosurgical generator is expected to have a usable lifetime extending many years and will not usually be replaced with a newer model for many years.